Reverse osmosis membranes are enclosed in a container or vessel when in use. When the membrane is utilized for desalination or, more correctly, for providing potable water there is brine or brackish water in the membrane and on the interior of the vessel. Salt water or brackish water contains various micro-organisms or the like which, if given time to develop, can produce a rather repulsive growth, typically rather slimy black stuff which does not appear in the potable water outlet but which tends to deposit on the vessel and the membrane and tends to generally foul up the system when it is necessary to service the membrane.
The slimy, algae-like growth has an offensive smell and appearance which can be tolerated under service conditions if that is all there is to it. But, as this growth develops it develops the ability to attack stainless steel of a grade which in normal use in salt water remains quite clean and serviceable. The slimy deposit attacks and pits the stainless steel and has limited the use of stainless steel in membrane containers even though such vessels would offer many advantages if they can be kept free of the pitting.